A large number of semiconductor gas sensors are presently in use in many parts of the world largely to provide early warning of the development of an explosion hazard (e.g. escaping flammable gas) or the presence of toxic gases or vapors in ambient air.
A sensing element normally comprising a semiconducting material and presenting a high surface-to-bulk ratio is deployed on a heated substrate between two metallic electrodes. The presence of gas posing a hazard is detected by a sensible change in the resistance of the semiconducting element by means of the electrodes that are incorporated in a suitable electric circuit. The device is thus a gas-sensitive resistor.
The most commonly used material in gas sensitive resistors used to measure impure gases in air is tin dioxide. Tin dioxide sensors, though often useful in particular alarm functions, have generally been found to suffer from a lack of selectivity.
The reactions that allow the detection of target gases normally involve the oxidation of the target gas at the semiconductor (oxide) surface and a concomitant change in the charge carrier density of the material. Unfortunately, changes in relative humidity also give rise to a sensible change in the conductivity of tin dioxide even though, in this case, no oxidation process is possible. In other words, changes in relative humidity amount to an interference with the detection of gases by tin dioxide even though the mechanisms involved in the two responses are different.
Since the reactions that generate the resistance response take place at the oxide surface, a very small amount of second phase additive may modify the behavior substantially.